What Causes Unstable Engine Idle Speed with Fluctuating RPM?

[DeAnnie]

Engine idle speed instability with fluctuating RPM is caused by: oil accumulation and carbon buildup in the throttle body, excessively dirty intake passages, fuel system blockages (particularly in fuel injectors), and intake manifold malfunctions. Unstable idle speed is one of the most common engine faults, ranging from minor to severe issues. Normally, engine idle speed should remain constant. Generally, higher idle speeds lead to increased fuel consumption, while excessively low idle speeds may cause engine vibration or even stalling. Oil accumulation and carbon deposits in the throttle body, along with dirty intake passages: Blocked intake passages can cause idle speed fluctuations. Carbon buildup or oil presence in the throttle body is a frequent cause of idle instability. Throttle body carbon deposits affect its opening angle, disrupting air intake volume. This results in erratic RPM fluctuations during cold starts, accompanied by noticeable vibration.